What happens next with special counsel Robert Mueller and President Trump?

Clearly, White House Counsel Donald McGahn or his associates were the source of the explosive leak that Trump had directed McGahn to fire Mueller, and that McGahn blocked the move by threatening to resign. But why did McGann choose last week to leak the story of an episode that happened last June?

The most obvious explanation is that Trump was again making noises about firing Mueller. If that was the concern, McGahn's leak worked, with several key Republicans again warning that firing the special counsel is a red line that Trump can't cross.

Mueller is expected to interview Trump in the next few weeks. That's a sign that Mueller is close to filing his report. The way this works is that you wait until you have all the goods on the ultimate target from other interviews and investigations, and then you bring in Mr. Big last. If Mueller is ready for Trump, that's a sign he's wrapping up.

The report, when filed, is likely to be devastating in its detail. Far more details on Trump's efforts to kill former FBI Director Comey's investigation; evidence of complicity by Trump's family and perhaps by Attorney General Jeff Sessions and possibly by Vice President Pence; lots more detail on how Russian money has been propping up a failing Trump real-estate empire for more than a decade, and detail on what the Russians expected and got in return.

In short, more than enough to impeach Trump.

And then, House and Senate Republicans will go into a huddle and decide whether it's time for him to go—whether to advise him to resign in exchange for a deal with Mueller, perhaps sparing his family.

Republican leaders will likely be split on this. Throwing Trump under the bus will infuriate his hard-core base. But keeping Trump in the face of overwhelming evidence of impeachable offenses will make the 2018 elections a cakewalk for Democrats.

And one more complication: If Vice President Pence is implicated, then both men might have to resign or be impeached. And that creates an intriguing set of choices for one Paul Ryan, House Speaker—and next in line for the presidency.